Your Session has been expired. Kindly close the browser and Login again through GAIL's website.
This error has occured for one of the following reasons :
1) You have used Back/Forward/Refresh button of your Browser.
2) You have double clicked on any options/buttons.
3) You have kept the browser window idle for a long time.
4) You have tried to access the link directly. You must access the application only through GAIL's website.